A few comments on the contenders:
Sony is a company that I despise. They expressed open contempt for their U.S. customers years ago. They shipped pop music audio CDs that install a nasty root kit software on any Windows computer on which the CD is played. While I don't buy any pop music, I don't care for such business practices and I avoid their products whenever possible. Minolta was a company with a fine tradition and great innovation in photography. It was a shame to see them exit the business.
Olympus is also a company with great tradition in photography, especially in the compact OM series. They always made great lenses, and still do. The 4/3 system has some great technical innovation, but the performance comes up short in terms of weak high ISO performance, mediocre AF, and limited choice of lenses an accessories. And the high end lenses are very expensive. I have to pass on them.
Canon has evolved into a big company with lots of different products, including copiers and printers as well as cameras. They certainly tried to innovate to compete against the market leader Nikon for many years, and they have come up with some impressive products. The better Canon bodies and the L series lenses are nicely made, perform very well, and have a wide range of lenses to choose from. I never cared for the "Rebel" line (starting with the original Rebel film camera) as they seem to be poorly made plastic-y stuff. They work fine but they just don't feel nice in the hand. Canon is certainly a contender for most photography applications.
Nikon has stuck to the core business of photography to a greater degree than Canon. Canon has a broader range of photography products, but the Nikon products are generally equal to the Canon products in terms of price and performance where there are similar products. Nikon has far fewer fixed focal length lenses, but some of the best "pro quality" f2.8 zooms in the business, including the 14-24 f2.8 AF-S FX and the 24-70 f2.8 AF-S FX.
A year ago I decided to switch systems and I was faced with the choice of Canon or Nikon. I'm sure either would fit my needs, but my specific concern was a system to handle fast action sports and better low-light AF and image quality. I also decided to go with a few choice f2.8 zooms rather than a bunch of primes. I decided to go with Nikon for the following reasons: 1. I liked the ergonomics of the Nikon bodies better. I found the controls on the Canon bodies to be less logical and less convenient. Why does Canon put a dedicated "print" button on an advanced DSLR? 2. For the specific lenses I had selected, I liked the build quality of the Nikon lenses better. Compare the Nikon 17-55mm f2.8 VR to the Canon 17-55mm f2.8 EF-S lenses side by side, or the Nikon 70-200mm f2.8 VR to the similar Canon IS lens, or the Nikon Micro 105mm f2.8 VR comared to the Canon Macro 100m f2.8. These were the lenses I was buying, and I just liked the feel and handling of the Nikon gear better. 3. Low-light performance of the Nikon cameras seemed better, and that lead has increased with the D3/300/700.
I was a close call but Nikon seemed better for me. I don't regret the choice but I would probably been satisfied with Canon as well.
Originally posted by AndrewG NY I assume you're really talking about the canon 100 f/2.8 macro. That sounds like a challenge that I'm up for should anyone want to send me one with a body to mount it on--I'm pretty certain I could take a bad picture with any equipment. While USM is probably a nice feature to add to ordinarily ponderously focusing macro lenses, and I'm sure the Canon you mention is a nice optic, other macro lenses are already so good that I find it very hard to believe that this one trounces the rest to the point where someone should seriously consider picking a system around it.
I find my Micro Nikkor 105mm f2.8 VR to be more than equal to anything in this class. The Silent Wave Motor and Vibration Reduction don't add anything for macro shots, but they are a pleasure for general use. The close-up image quality is fantastic. It's beautifully built with a metal body and deep, silky focusing ring, and a sturdy deep hood. Only drawback is that it a big, heavy beast. If you take a bad picture with this lens it's your own fault. But in all honesty, the shots with a Pentax or Canon (or even a Sigma) 100mm macro are probably just as good, and those lenses are significantly cheaper and lighter.